1. Field of the Invention
The present invention generally relates to an X-ray imaging apparatus for converting pulsatory X-ray beams penetrating through a biological body under medical examination into optical X-ray images and also for converting the optical X-ray images into X-ray image signals to obtain visual X-ray images of the biological body. More specifically, the present invention is directed to such an X-ray imaging apparatus that electron charges produced only by the desirable pulsatory X-ray beams can be derived from an image sensor as effective X-ray image signals with better image qualities.
2. Description of the Prior Art
In the X-ray medical diagnostic field, such an X-ray imaging system has been utilized that X-ray beams are projected to a biological body under medical examination, and the X-ray beams which have passed through the biological body are supplied to an image intensifier so as to obtain an optical X-ray image, and thereafter this optical image is converted into an X-ray television signal to be displayed on a TV monitor. Very recently, to improve image qualities of these X-ray images, pulsatory X-ray beams are extensively utilized in these conventional X-ray imaging systems. This is because when the widths of the pulsatory X-ray beams are shortened as short as being permitted, a blurring phenomenon caused by movements of a biological body can be considerably reduced, or suppressed. A typical waveform of such a pulsatory X-ray beams is represented in FIG. 1. It should be noted that in case of low X-ray density, unwanted X-ray trailing edges, or lags "L" necessarily appear after the respective pulsatory X-ray beams having normal pulse widths (namely, ideal X-ray pulses). X-ray images produced from these unwanted trailing edges "L" have a so-termed "blurring phenomenon", resulting in deterioration of X-ray image qualities.
In FIG. 2, there is shown the arrangement of the conventional X-ray imaging system, as described in, for instance, U.S. Pat. No. 4,612,572 to KOMATSU et al. The X-ray imaging system includes an X-ray tube 1 for emitting pulsatory X-ray beams 12; a biological body 2; an image intensifier unit 3 for converting pulsatory X-ray beams 12 which have penetrated through the biological body 2 into optical image signals; an image pick-up tube, or television camera 4 for converting the optical image signals into electronic image signals; and also a television monitor (CRT) 5 for displaying X-ray images of the biological body in response to the electronic image signals. The X-ray imaging system further comprises a system controller 7 for controlling the entire circuit arrangement; an X-ray generator 8 for sending an emission control signal to the X-ray tube 1 so as to control generation of the pulsatory X-ray beams 12; an I.I. (image intensifier) blanking unit 9 for blanking the unnecessary X-ray images caused by the above-explained X-ray trailing edges "L", which will be inputted into the receiving plane of the I.I. unit 3; and a television sync signal generator 10 for supplying a television sync signal to the image pick-up tube 4.
To prevent such unnecessary X-ray images caused by the X-ray trailing edges "L" from being entered into the I.I. unit 3, there is provided the I.I. blanking unit 9 which has such a function that the electron charges produced by the X-ray trailing edges within the I.I. unit 3 are not converged onto the image focusing plane of the I.I. unit 3.
The above-described conventional X-ray imaging system has a drawback such that the I.I. blanking unit 9 is necessarily required so as to avoid the adverse influences given to the image qualities due to occurrences of the pulsatory X-ray lags "L". Accordingly, the X-ray imaging system costly becomes high and also large in size.
There is another problem in the conventional X-ray imaging system that since the response time of the I.I. blanking unit is relatively slow, e.g., 2 to 3 milliseconds, the operable maximum speed of this X-ray apparatus should be limited, or lowered.